1. Field of the Invention
This invention relates to ferroelectric liquid crystal (FLC) devices, and particularly to a method and apparatus for driving the liquid crystal elements of such devices.
2. Description of Related Art
A ferroelectric liquid crystal has a permanent electric dipole which interacts with the applied electric field. Hence, ferroelectric liquid crystal elements exhibit fast response times, which make them suitable for use in display, switching and information processing applications. In particular, FLC displays will provide important alphagraphic flat panel displays for office applications.
The stimulus to which the FLC element responds is a dc field, and its response is a function of the applied voltage (V) and the length of time (t) for which the voltage is applied. The element is switched to one state by the application of a voltage of a given polarity across its electrodes, and is switched to the other state by the application thereto of a voltage of the opposite polarity. It is essential that an overall dc voltage shall not be applied across such an element for an appreciable period, so that the elements remain charge-balanced, thereby avoiding decomposition of the liquid crystal material. Pulsed operation of such elements has therefore been effected, with a pulse of one polarity being immediately followed by a pulse of the other polarity, so that there is no resultant dc polarisation.
The liquid crystal elements are commonly arranged in matrix formation and are operated selectively by energising relevant row and column lines. Time-division multiplexing is effecting by applying pulses cyclically to the row (strobe) lines in sequence and by applying pulses, in synchronism therewith, to the column (data) lines.
It is known that the electronic waveforms used to drive a ferroelectric liquid crystal display (FLCD) affect greatly the contrast ratio and the frame time of such a display. Hence, these waveforms will have a great impact on the commercial exploitation of ferroelectric LCDs.
FIGS. 1(a), 1(b) and 1(c) of the accompanying drawings illustrate the waveforms occurring in one known FLCD drive scheme. FIG. 1(a) shows the waveform for one row of devices of the display. The waveform 1 comprises a positive pulse 2 of amplitude V.sub.s followed immediately by a negative pulse 3 of the same amplitude. After a delay 4, a further negative pulse 5 of amplitude V.sub.s is followed immediately by a positive pulse 6 of amplitude V.sub.s. FIG. 1(b) shows a corresponding section of a "non-select" column waveform 7. That section comprises a positive pulse 8 of amplitude V.sub.D immediately followed by a negative pulse 9 and, after a delay 10, a negative pulse 11 immediately followed by a positive pulse 12. The pulses 9, 11 and 12 are all of amplitude V.sub.D. The pulses 8, 9, 11 and 12 are of the same width as, and are synchronized with, the pulses 2, 3, 5 and 6. Corresponding column waveform sections for the other rows will occur during the delay period 10. Alternatively, a corresponding section of a "select" column waveform 13 comprises pulses 14-17 of the opposite polarities to the pulses 8, 9, 11 and 12. This scheme uses two sets of bipolar pulses to achieve the desired switching and is, therefore, called a "four-slow" scheme. It is now known that that scheme gives rise to low contrast and long frame times. The frame time is given by the pulse width (t.sub.s1) .times. number of slots .times. number of rows in the display. The frame time can be halved by splitting the column electrodes in half and driving the resulting two sets of row electrodes in parallel.
A much reduced frame time can be achieved by using a "two-slot" scheme as disclosed in our British Patent Publication No: 2,208,559A, which scheme is illustrated in FIG. 2 of the present drawings. In this case the strobing (row) signal (FIGS. 2(a), 2(b), and 2(c)) comprises a positive pulse 20 of amplitude V.sub.s, followed by a negative pulse 21 of amplitude V.sub.s ', which is less than V.sub.s. This is the only pair of strobe pulses occurring during a frame period. The corresponding data (column) signal section comprises either a positive pulse 22 followed by a negative pulse 23 (FIG. 2(b)) or a negative pulse 24 followed by a positive pulse 25 (FIG. 2(c)), depending upon the data to be written. The pulses 22-25 are all of amplitude V.sub.D (not necessarily equal to V.sub.D of FIG. 2). The width of each pulse is t.sub.s2.
Since the strobe pulses 20 and 21 are of different amplitudes, there would be a residual dc level applied to the addressed liquid crystal elements and, as stated above, this is undesirable. A small dc voltage V.sub.G is therefore applied to the strobe line between the end of the pulse 21 and the beginning of the pulse 20 of the next frame period. The required voltage V.sub.G is given by ##EQU1## where N is the number of rows.
Although the known scheme of FIGS. 2(a), 2(b) and 2(c) can have half the frame time of the FIGS. 1(a), 1(b) and 1(c) scheme, the contrast ratio achieved by the FIGS. 2(a), 2(b) and 2(c) scheme is generally similar to that obtained by the FIGS. 1(a), 1(b) and 1(c) and can be low, for example .ltoreq.5:1.
A further known scheme is illustrated in FIGS. 3(a), 3(b) and 3(c) of the drawings. In this case the strobe signal 30 (FIG. 3(a)) comprises a negative pulse 31 of amplitude V.sub.s and a positive pulse 32 also of amplitude V.sub.s. The corresponding "non-select" column signal section 33 (FIG. 3(b)) comprises a negative pulse 34 occurring just before the pulse 31, immediately followed by a positive pulse 35 aligned with the pulse 31. A positive pulse 36 is then followed immediately by a negative pulse 37 aligned with the pulse 32. The "select" column signal section 38 (FIG. 3(c)) comprises pulses 39-42 aligned with, but of opposite polarity to, the pulses 34-37, respectively. All of the pulses 34-37 and 39 to 42 are of amplitude V.sub.D (not necessarily equal to V.sub.D of FIGS. 1(a), 1(b) and 1(c) of FIGS. 2(a), 2(b) and 2(c) , and each of these pulses, as well as each of the pulses 31 and 32, is of width t.sub.s3.
If the schemes of FIGS. FIGS. 1(a)-1(c), FIGS. 2(a)-2(c) and FIGS. 3(a)-3(c) are compared, it is found that t.sub.s1 .apprxeq.t.sub.s2 .apprxeq.t.sub.s3. The scheme of FIGS. 3(a), 3(b) and 3(c) therefore operates with short pulse width and has the advantages of short switching times and high contrast ratio, but the disadvantages of being a four-slot scheme, which leads to a long frame time.
The known schemes can therefore achieve either a high contrast ratio or a short frame time, but none can achieve both of these desirable features together.